Clock signals in an integrated circuit (IC) may be generated by a clock circuit including a crystal oscillator and one or more phase-locked loops (PLLs). An external piezoelectric quartz crystal, tuned for a specific parallel-resonant frequency, may be electrically resonated by the on-chip crystal oscillator. The crystal frequency, called the reference frequency (fREF), may be fed to a PLL which produces the desired output frequency.
The PLL multiplies the reference frequency to the desired output frequency. The PLL locks the output frequency to the reference frequency. If the reference frequency moves, the output frequency will track the move.